In NC you probably don't need more than than a 3-sided shed. I have a book on barn building and it suggests such a building for warmer climates. I kept my horses outside with just a shelter in Central IL for a full 14 years. I couldn't WAIT to have a barn with stalls and good storage, even though I had to construct the stalls myself.
You NEVER KNOW when a horse will get sick or injured, or you get a new animal and have to separate the herd. You prevent injuries from the ice by having stalls, too, but in NC that probably isn't an issue. This week the temps dipped low and I added straw to everybody's stall. I also keep blankets, but they usually just get dusty--but they are there all the same.
IMHO, you can't have too many buildings with a roof. A barn is a real prize.